1. Field of the Invention
The present invention relates to a belt conveying device, and particularly, to various types of belt conveying devices for use in an image forming apparatus, such as a copy machine and a printer, and an image forming apparatus provided with the same.
2. Description of the Related Art
As a conventional electrophotographic image forming apparatus, there is known an image forming apparatus that uses an intermediate transfer belt that is a transfer medium. There is also known an image forming apparatus that uses an endless conveyor belt as a conveying unit of a recording paper sheet that is a transfer medium. The belt used in these apparatuses is stretched by a plurality of rollers and is driven to rotate. During the rotation, there sometimes occurs a belt shift, in which a belt position moves in a direction (main scanning direction) perpendicular to a belt conveying direction, and also occurs a belt skew, in which the belt conveying direction is inclined toward the main scanning direction.
The occurrence of the belt skew causes a displacement of an image forming position on a transfer medium such as the intermediate transfer belt or the recording paper sheet, and this causes a distortion of an image. Moreover, in a color image forming apparatus that forms single-color images of black (hereinafter, “Bk”), yellow (hereinafter, “Y”), magenta (hereinafter, “M”), and cyan (hereinafter, “C”) and superimposes these images on one another on the transfer medium to obtain a color image, each displacement between image forming positions results in a color misregistration between toner images of the respective colors. Because these lead to degradation in image quality, some measures have to be taken for the belt skew in order to obtain a high-quality image.
Various methods are proposed to deal with the above-mentioned problem. One method is to provide a shift guide member on an endless belt. In this method, the shift of the endless belt is suppressed by bringing the shift guide member provided on the surface of the belt into contact with an end face of a belt conveying roller so as to regulate the force in the main scanning direction produced in the belt. However, the belt skew caused by the deflection of the shift guide member in the main scanning direction or the deflection of the end face of the belt conveying roller cannot be suppressed and thus an image distortion or a color misregistration may occur.
Japanese Patent Application Laid-open No. 2005-148127 discloses a configuration that uses a shift guide member provided on a belt to regulate the belt shift. In the configuration, a position of a latent image formed on a photosensitive element is controlled based on a previously measured meandering component for one rotation of a belt.
Other than the method of providing the shift guide member on an endless belt, Japanese Patent Application Laid-open No. 2006-276427 discloses a configuration that uses a skew detector. When the skew of the endless belt is detected, i.e., the state where the endless belt is conveyed while being inclined with respect to the belt conveying direction is detected by the skew detector, an image distortion is corrected by an image forming unit based on a skew amount of the endless belt detected by the skew detector.
Moreover, for velocity fluctuation in a belt conveying direction (sub-scanning direction), there is known a technology for reading marks continuously formed on the surface of the belt at predetermined intervals by a mark detector, calculating a conveyance velocity of the belt from a time interval of mark reading signals output from the mark detector, and controlling the rotation of a motor rotating a drive roller based on the calculated velocity so that the conveyance velocity of the belt becomes a predetermined velocity. According to Japanese Patent Application Laid-open No. 2002-323806, by controlling the generation of an image signal of a latent-image forming unit, a color misregistration due to the displacement between image forming positions along the sub-scanning direction can be largely reduced. Furthermore, Japanese Patent Application Laid-open No. 2008-129518 discloses a belt conveying device that uses a 2D sensor for detecting a movement of a belt in the sub-scanning direction and a position of the belt in the main scanning direction. The belt conveying device performs a feedback control and a feedforward control based on a position of the belt in its shift direction.
However, there are the following problems in the method, disclosed in Japanese Patent Application Laid-open No. 2005-148127, of suppressing the belt shift by providing the shift guide member on the endless belt and suppressing an effect of belt meandering, which cannot be suppressed by the guide member, by controlling the latent-image forming position on the photosensitive element.
Because the meandering component for one rotation of the belt, which includes the meandering component caused by the deflection of the end face of the belt conveying roller and the deflection of the shift guide member in the main scanning direction, is previously measured, the belt meandering caused by the deflections of both the roller end face and the guide member can be suppressed. However, to deal with a deformation of the endless belt and the shift guide member over time and a deformation in association with changes in environment such as temperature and humidity, it is necessary to frequently measure the meandering component. This causes frequent interruption of the image forming operation, which largely hinders an increase in speed of image output. Moreover, there is also a problem in that dealing with a dynamic deformation due to effects of vibration or the like is difficult. In addition, in the method of suppressing the belt shift by providing the shift guide member on the endless belt, large external force is applied to the shift guide member when the belt is driven at a high speed, which causes buckling and breakage of the belt and the shift guide member. This makes it difficult to speed up image output.
Furthermore, the method, disclosed in Japanese Patent Application Laid-open No. 2006-276427, of controlling the latent-image forming position on the photosensitive element by detecting the belt skew without providing the shift guide member on the endless belt has the following problems.
In order to detect the belt skew, there is a first method that uses a control signal of a steering roller. The steering roller corrects the belt shift by controlling the inclination of the belt such that each portion on the belt always passes the same position after each rotation of the belt. The skew during one rotation is determined depending on inclined states of the rollers. The belt skew in an image transfer surface, to which an image is transferred from the photosensitive element, that affects an image distortion and a color misregistration is not always determined only by the inclination of the steering roller. If any roller other than the steering roller is inclined by temperature change or its variation over time, the belt skew cannot be accurately detected by the control signal of the steering roller. Thus, it is impossible to accurately correct the belt skew.
In order to detect the belt skew, there is a second method that forms a mark, which is used for detecting an image position, on a non-image area of an intermediate transfer belt and detects the mark by a mark detector. However, this method has a problem in that it is necessary, in order to form the mark for detecting an image position, to develop a latent image of the mark formed on the photosensitive element by toner and to transfer the developed latent image to the intermediate transfer belt. A large amount of toner is consumed in order to always detect the belt skew, which causes an increase in image formation cost. An ordinary image is transferred from the intermediate transfer belt to a recording sheet such as a paper sheet. In contrast, the mark for detecting an image position is not transferred and has to be removed by a cleaning member, which causes the load of intermediate-transfer-belt cleaning to increase and also may cause a cleaning failure.
In order to detect the belt skew, there is a third method that detects a belt edge at a plurality of positions along the belt conveying direction on the image transfer surface to which an image is transferred from the photosensitive element. However, unlike the conventional case in which only one belt edge sensor is provided to control the steering roller, this method requires a plurality of belt edge sensors, which causes an increase in cost. In addition, it is difficult in terms of layout that the belt edge sensors are arranged in the belt conveying direction of the image transfer surface along which a plurality of photosensitive elements for full-color image formation is arranged. This causes an enlarged device size due to an increase in a belt circumferential length and a cost increase.
Because the belt edge is detected as a signal for controlling the belt shift by the steering roller, edge data detected by the edge sensor shows a shape that includes a shape of the belt edge. To control meandering of the belt by referring to the previously measured edge data, it is necessary to synchronize, the belt. Thus, a detection signal from a belt home sensor for detecting a home position of the belt is required. Furthermore, there may occur so-called “belt meandering” in the intermediate transfer belt, by which the belt moves in the direction perpendicular to the belt conveying direction due to a manufacturing error of the intermediate transfer belt. A control is performed so that the “belt meandering” is prevented and edge positions of the intermediate transfer belt are made constant by detecting the edges of the intermediate transfer belt by the edge sensor and controlling the steering roller in accordance with the edge positions of the intermediate transfer belt detected by the edge sensor. In this case, even if the edge positions of the intermediate transfer belt are made constant, the “belt meandering” may remain if the edge positions of the intermediate transfer belt themselves are periodically displaced from predetermined positions. Thus, the periodic displacements of the edge positions of the intermediate transfer belt are previously detected by the edge sensor in a state in which the steering roller is not controlled. The data obtained by averaging the periodic displacements of the edge positions of the intermediate transfer belt is stored in a storage unit provided in a steering control circuit or the like. The displacements of the edge positions of the intermediate transfer belt that actually occur are detected based on displacement information for the edge positions of the intermediate transfer belt stored in the storage unit in order to prevent the “belt meandering”. Because of these factors, an increase in device cost is inevitable.